1. Field of the Invention
The present invention relates to an image display technique.
2. Description of the Related Art
Conventionally, some Head Mounted Displays or Hand Held Displays, which display a virtual image by guiding image light to the pupils of an observer, have been proposed. Of these displays, an image display apparatus which displays an image based on light rays reflected by an oscillating reflecting surface is disclosed in patent reference 1.
The image display apparatus disclosed in patent reference 1 includes one horizontal scanning unit (common to the right and left eyes) and two vertical scanning units. Light beams coming from two light sources for the right and left eyes are scanned by the horizontal and vertical scanning units via collimator lenses, and are guided to the eyeballs via optical systems.
An image display apparatus disclosed in patent reference 2 uses a two-dimensional scanning mirror, and attains an optical size reduction.
In case of a MEMS (Micro Electro Mechanical Systems) mirror which uses the resonance phenomenon of a silicon substrate, the Young's modulus of the silicon substrate changes due to a change in temperature, and the resonance characteristic (maximum resonance frequency) changes accordingly. For this reason, assuming that the driving frequency is constant (the frequency of an AC voltage to be applied is constant), the deflection angle of a micromirror varies due to a change in temperature, thus posing a problem. For example, even when the temperature of the micromirror has changed, the oscillating frequency does not change unless the frequency of an AC voltage to be applied is changed. However, as the temperature of the micromirror rises, the deflection angle of the micromirror becomes smaller. In this way, when the deflection angle of the micromirror varies, the scanning range of reflected light of a light beam irradiated toward the micromirror varies. Therefore, if there are a time period in which both light beams for the right and left eyes are emitted and a time period in which they are not emitted, the temperature of the MEMS mirror varies along with an elapse of time, and such temperature variation appears as an image distortion.
Techniques that solve the problems caused by the change in resonance frequency are disclosed in patent references 3 and 4.
An optical scanning apparatus disclosed in patent reference 3 is as follows. That is, based on the waveform of a back electromotive force generated in a coil equipped on one of a movable plate and main body depending on the oscillating state of the movable plate which is axially supported by the main body to be oscillatable, a sinusoidal exciting current to be supplied to the coil is stopped for a predetermined period. During this period, the resonance frequency is detected. After that, the movable plate is driven by the sinusoidal exciting current of the detected resonance frequency, thus controlling the movable plate to follow a change in resonance frequency, and stabilizing the amplitude.
An optical scanning apparatus disclosed in patent reference 4 measures the deflection frequency upon sinusoidal oscillation of the deflection plane of a deflection unit that deflects a light beam, and adjusts a time period from the end of scanning of a forward path until the start of scanning of a backward path in accordance with the measured deflection frequency. Then, a light beam emitting unit is controlled so that a time period from the start of scanning until the end of scanning of the forward path matches that from the start of scanning until the end of scanning of the backward path.
[Patent Reference 1] Japanese Patent Laid-Open No. 11-95144
[Patent Reference 2] Japanese Patent Laid-Open No. 2006-162780
[Patent Reference 3] Japanese Patent Laid-Open No. 2003-177347
[Patent Reference 4] Japanese Patent Laid-Open No. 9-230277
However, the aforementioned related arts suffer the following problems.
As is known, the MEMS mirror suffers variations of the resonance characteristics of individual devices even when it is a device produced by the processes under identical conditions in the manufacturing process or a device formed on a single substrate. That is, even when the manufacturing process and operating condition remain the same, individual MEMS mirrors which are being driven have different resonance frequencies. Therefore, when an image display apparatus which displays images using a plurality of MEMS mirrors is to be implemented, a large difference is often generated between the resonance frequencies of the individual MEMS mirrors due to a change in resonance frequency caused by a change in operating temperature at the time of driving in addition to variations in the manufacturing process. When the image display apparatus using the plurality of MEMS mirrors adopts respective units disclosed in patent references 3 and 4, it is possible to optimally drive individual micromirror scanners to follow changing resonance frequencies. However, the individual MEMS mirrors are driven at different driving frequencies.
For example, in an image display apparatus which guides deflected beams by different MEMS mirrors to the right and left eyes, different display timings of right and left images due to different refresh rates of right and left images result in unnatural feeling experienced by an observer. In an image display apparatus which makes a tiling display that forms one frame by combining individual images formed by deflected beams guided by a plurality of MEMS mirrors, the boundaries between a plurality of images becomes conspicuous due to different refresh rates. As a result, the observer feels an unnatural sensation.